This invention relates in general to hybrid motor vehicles operable by either an electric motor or a liquid fuel powered internal combustion engine and, in particular, to a method and system for reducing the cycling of the engine during non-propulsive operations.
For various reasons, including environmental reasons, it is desirable to provide automotive vehicles that operate with propulsion systems other than the typical internal combustion engine. One such propulsion system contemplated is a purely electric vehicle. However, because of well known problems associated with such electrical vehicles, combining the electric drive with a somewhat more conventional internal combustion engine is one alternative being considered. A vehicle with such an alternative propulsion system is generally referred to as a hybrid motor vehicle.
A hybrid electric vehicle (HEV) is generally described as a motor vehicle with a main power unit (HPU) which converts fuel energy to electric and/or mechanical energy, and a bidirectional energy storage system (ESS), usually electrical. The main power unit may be a piston engine, gas turbine, fuel cell or the like, while the energy storage system may be a battery, flywheel, capacitor or the like.
Motive power to drive the vehicle, as demanded by the driver, is drawn from a combination of these two sources. The essential elements of a hybrid vehicle powertrain include an HPU, an electric traction motor-generator, and an ESS. Various hybrid configurations add HPU and ESS power in different places in the system.
The emissions that occur at engine startup are at a high level due to the fact that the air/fuel ratio is not accurately determined on startup and the catalytic converter is usually cold (unless it is electrically heated). Upon startup, initial emissions are high due to low catalyst efficiency until the catalyst temperature is stabilized. It is, therefore, desirable to limit the frequency of engine starts in a given drive cycle. Emissions are much lower if the engine is requested to remain on for a short extended period beyond driver demand (accelerator) to satisfy ancillary requirements. During this extended run period, the catalyst is sufficiently hot from the ongoing combustion process to perform the catalytic action required to maintain low emissions.
Ancillary functions that require engine operation include, but are not limited to: brake booster vacuum, fuel vapor purge, and passenger compartment heating or air conditioning. It is an object of the present invention to reduce the number of times that the engine or other main power unit is turned on solely for supporting non-propulsive or ancillary functions to thereby enhance both fuel economy and reduce engine emissions.
In accordance with the present invention, a method and system is provided that responds to non-propulsive needs of a hybrid vehicle by setting at least two thresholds related to engine support: a first threshold to keep a running engine ON, and a second threshold to turn the engine ON from an OFF state. More particularly, a request is made that the engine not be turned OFF until the value of an auxiliary system parameter exceeds the first threshold value and that the engine be turned ON if the parameter falls below the second threshold value. In other words, if the engine is OFF when the value of the parameter falls below the second threshold value, a request is made that the engine be turned ON and whenever the engine is ON, a request is made that the engine be maintained ON until the parameter exceeds the first threshold value.
The logic expressed above will extend engine run time slightly but will reduce the frequency of engine startups. This can be calibrated for a clear emissions improvement and a modest fuel economy improvement.